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NASA NTRS1988

Polarimetric microwave scattering from known randomly rough surfaces

Scattering from rough surfaces is reviewed, and artificial randomly rough surfaces are described. Measurements were made from 4 to 18 GHz for the 4 linear polarization states. The target consists of a metalized randomly rough surface constructed from known surface statistical information. The surface was generated using surface generation routines on the computer, and then the data was converted for the machining of the surface on a computer controlled mill. Bistatic and monostatic measurements were acquired at incident angles from 25 to 60 deg. At the 13.5 to 18 GHz frequencies, the effects of backscatter enhancement and depolarization effects can be studied in a regime which does not satisfy Kirchoff or small perturbation assumptions. This allows scattering mechanisms to be introduced into models, which may show missing elements in the more standard formulations. At lower frequency bands such as 4 to 8.5 GHz the scattering behavior was also examined to see the behavior as assumptions of other formulations are satisfied.

Nance, C. E.↗

NASA NTRS1988

Interactions of light with rough dielectric surfaces - Spectral reflectance and polarimetric properties

The nature of the interactions of visible and NIR radiation with the surfaces of rock and mineral samples was investigated by measuring the reflectance and the polarization properties of scattered and reflected light for slab samples of obsidian and fine-grained basalt, prepared to controlled surface roughness. It is shown that the degree to which radiation can penetrate a surface and then scatter back out, an essential criterion for mineralogic determinations based on reflectance spectra, depends not only upon the composition of the material, but also on its physical condition such as sample grain size and surface roughness. Comparison of the experimentally measured reflectance and polarization from smooth and rough slab materials with the predicted models indicates that single Fresnel reflections are responsible for the largest part of the reflected intensity resulting from interactions with the surfaces of dielectric materials; multiple Fresnel reflections are much less important for such surfaces.

Yon, S. A.↗

NASA NTRS1988

Influence of land surface roughness on atmospheric circulation and precipitation - A sensitivity study with a general circulation model

The influence of land surface roughness on the large scale atmospheric circulation and rainfall is examined by comparing three sets of simulations made with a general circulation model in which the land surface roughness length, z(0), was reduced from 45 cm to 0.02 cm. It is found that the reduced surface roughness produced a two-fold increase in the boundary layer wind speed, a two-fold decrease in the magnitude of the surface stress, and almost no change in the surface evaporation and surface sensible heat flux. It is suggested that the height of the earth's vegetation cover has a large influence on the boundary layer water vapor transport convergence and the rainfall distribution.

Sud, Y. C.↗

NASA NTRS1989

Characterization of Venera 15/16 geologic units from Pioneer Venus reflectivity and roughness data

The characterization of geologic units defined for the Venus surface on the basis of Venera 15/16 image data is presently conducted in light of Pioneer Venus reflectivity and rms slope data. Four geologic-unit groups are distinguished: smooth rocky units, rough ones, rough high-dielectric units, and diffusely-scattering ones. Models have been tested for the surface-property origins of several units, and it is determined that plains and tectonic units may be contrasted in terms not only of surface roughness but of surface-roughening tectonic deformation that appears to dominate the erosional regime of Venus.

Bindschadler, D. L.↗

NASA NTRS1990

A rough-surface thermophysical model for airless planets

A model for determining diurnal temperatures in spherical-section depressions and which encompasses both subsurface heat-flow and direct and scattered sunlight effects is presently applied to the disk-integrated thermal emission of a rough planetary surface with nonzero thermal inertia. Attention is given to the variation with roughness and thermal inertia of the beaming parameter eta, which characterizes zero-phase thermal emission by comparison with a smooth, nonrotating body and is almost independent of albedo for a given surface roughness. The thermal phase curve of Ceres is noted to be well matched by the model features of (1) prograde rotation, (2) 44-deg rms surface slope, and (3) a thermal inertia that is 30 percent of the lunar value.

Spencer, John R.↗

NASA NTRS1990

On the modelling of roughness effects

It is thought that the presently available numerical codes could be used for analysis of the laminar-turbulent transition process in boundary layers above rough walls with roughness simulated by proper distribution of surface suction. It is shown that the equivalence between both types of flows, i.e., the flow with surface roughness and the flow with distributed suction, might not exist.

Floryan, Jerzy M.↗

NASA NTRS1991

Relationships between topographic roughness and aeolian processes

The interaction between winds and desert surfaces has important implications for sediment transport on Earth, Mars, and Venus, and for understanding the relationships between radar backscatter and aerodynamic roughness as part of the NASA Shuttle Imaging radar (SIR-C) Mission. Here, researchers report results from measurements of boundary layer wind profiles and surface roughness at sites in Death Valley and discuss their implications. The sites included a flat to undulating gravel and sand reg, alluvial fans, and a playa. Estimates of average particle size composition of Death Valley sites and arithmetic mean values of aerodynamic roughness are given in tabular form.

Greeley, Ronald↗

NASA NTRS1991

The adverse aerodynamic impact of very small leading-edge ice (roughness) buildups on wings and tails

Systematic experimental studies were performed to establish the aerodynamic impact of very small leading-edge simulated ice (roughness) formations on lifting surfaces. The geometries studied include single element configurations (airfoil and 3-D tail) as well as multi-element high-lift airfoil geometries. Emphasis in these studies was placed on obtaining results at high Reynolds numbers to insure the applicability of the findings to full-scale situations. It was found that the well-known Brumby correlation for the adverse lift impact of discrete roughness elements at the leading edge is not appropriate for cases representative of initial ice build up (i.e., distributed roughness). It was also found that allowing initial ice formations of a size required for removal by presently proposed deicing systems could lead to maximum lift losses of approximately 40 percent for single-element airfoils. Losses in angle-of-attack margin to stall are equally substantial - as high as 6 degrees. Percentage losses for multi-element airfoils are not as severe as for single-element configurations, but degradations of the angle-of-attack-to-stall margin are the same for both.

Lynch, Frank T.↗

NASA NTRS1992

Influence of aerodynamic roughness length on aeolian processes: Earth, Mars, Venus

The aerodynamic roughness length (z sub 0) is the height at which a wind profile assumes a zero velocity. The lower part of the atmospheric boundary layer will be impeded by friction with the surface. An increase in surface roughness will also increase the shear stress required to initiate particle entrainment by the wind. Bagnold (1941) estimated z sub 0 as being 1/30 of the mean particle size. In Nature, surface roughness is composed of nonerodible elements as well as sand-size erodible particles. To assess z sub 0 values as a function of terrain, field experiments were conducted to obtain wind profiles monitored over natural surfaces at 15 sites in the Mojave Desert, Death Valley, and Nye County, Nevada. These sites span a variety of arid-land terrains, including smooth playas, alluvial fans, and lava flows; z sub 0 values ranged from 0.0001 cm to 1 cm. These values were incorporated in a threshold model and a flux model to assess transport efficiency over such terrains in three planetary environments (Venus, Earth, and Mars), and for particle sizes ranging from 60-500 micron. Threshold and flux are a function of planetary environment, particle density and size (Dp), and z sub 0, and the shear velocity of 1.2 x U sub *t (for Dp = 250 micron and z sub 0 = 0.84). Results show that flux on Mars is approximately 14 g/(cm x s), on Earth it is approximately 3 g/(cm x s), and on Venus 0.5 g/(cm x s). Under all planetary environments, the results also show a dramatic decrease in the flux for particles greater than 200 microns when z sub 0 increases above 0.0085 cm (corresponding to sites consisting of alluvium). When z sub 0 approaches 0.03 cm (corresponding to a mantled pahoehoe lava), the flux diminishes.

Blumberg, Dan G.↗

NASA NTRS1991

Heat transfer in the turbulent boundary layer with a step change in surface roughness

Measurements of Stanton numbers, velocity profiles, temperature profiles, and turbulence intensity profiles are reported for turbulent flat plate boundary layer flows with a step change in surface roughness. The first 0.9 m length of the test surface is roughened with 1.27 mm diameter hemispheres spaced 2 base diameters apart in a staggered array. The remaining 1.5 m length is smooth. The experiments show that the step change from a rough to a smooth surface has a dramatic effect on the convective heat transfer. In many cases, the Stanton number drops below the smooth-wall correlation immediately downstream of the change in roughness. The Stanton number measurements are compared with predictions using the discrete element method with excellent results.

Taylor, Robert P.↗

NASA NTRS1990

Evaluation of sensible heat flux, latent heat flux, and effective aerodynamic roughness at the land surface from a combination of satellite and in-situ data

A simple method is presented for using the daytime rise of the PBL height, combined with coincident satellite skin temperature measurements, to evaluate sensible heating and the effective surface roughness for heat transfer at synoptic locations. Multiple runs of a combination surface layer-mixed layer model are made in the central U.S., varying the values of surface moisture and roughness length around climatological means, and initializing the model with a set of atmospheric conditions from the synoptic report. The results, shown in figures and tables, are quite reasonable and spatially coherent, and depict well the sharp change in surface moisture and roughness which is known to take place across the central U.S.

Diak, George R.↗

NASA NTRS1991

Boundary-layer receptivity of sound with roughness

An experimental study of receptivity was carried out using an acoustical disturbance in the freestream. The receptivity was enhanced by using a uniform two-dimensional roughness strip (tape). The roughness strip generated the local adjustment in the flow needed to couple the long-wavelength sound wave with the short-wavelength T-S wave. The method proved to be highly sensitive, with slight changes in the forcing frequency or in the height of the 2D roughness element having a strong effect on the amplitude of the observed T-S wave.

Saric, William S.↗

NASA NTRS1992

Surface roughness considerations for atmospheric correction of ocean color sensors. I - The Rayleigh-scattering component. II - Error in the retrieved water-leaving radiance

The first step in the Coastal Zone Color Scanner (CZCS) atmospheric-correction algorithm is the computation of the Rayleigh-scattering (RS) contribution, L sub r, to the radiance leaving the top of the atmosphere over the ocean. In the present algorithm, L sub r is computed by assuming that the ocean surface is flat. Calculations of the radiance leaving an RS atmosphere overlying a rough Fresnel-reflecting ocean are presented to evaluate the radiance error caused by the flat-ocean assumption. Simulations are carried out to evaluate the error incurred when the CZCS-type algorithm is applied to a realistic ocean in which the surface is roughened by the wind. In situations where there is no direct sun glitter, it is concluded that the error induced by ignoring the Rayleigh-aerosol interaction is usually larger than that caused by ignoring the surface roughness. This suggests that, in refining algorithms for future sensors, more effort should be focused on dealing with the Rayleigh-aerosol interaction than on the roughness of the sea surface.

Gordon, Howard R.↗

NASA NTRS1993

Surface roughness due to residual ice in the use of low power deicing systems

Thicknesses of residual ice are presented to provide information on surface contamination and associated roughness during deicing events. Data was obtained from low power ice protection systems tests conducted in the Icing Research Tunnel at NASA Lewis Research Center (LeRC) with nine different deicing systems. Results show that roughness associated with residual ice is not characterized by uniformly distributed roughness. Results also show that deicing systems require a critical mass of ice to generate a sufficient expelling force to remove the ice.

Shin, Jaiwon↗

NASA NTRS1992

Receptivity of a laminar boundary layer to the interaction of a three-dimensional roughness element with time-harmonic free-stream disturbances

Receptivity of a laminar boundary layer to the interaction of time-harmonic free-stream disturbances with a 3D roughness element is studied. The 3D nonlinear triple-deck equations are solved numerically to provide the basic steady-state motion. At high Reynolds numbers, the governing equations for the unsteady motion are the unsteady linearized 3D triple-deck equations. These equations can only be solved numerically. In the absence of any roughness element, the free-stream disturbances, to the first order, produce the classical Stokes flow, in the thin Stokes layer near the wall (on the order of our lower deck). However, with the introduction of a small 3D roughness element, the interaction between the hump and the Stokes flow introduces a spectrum of all spatial disturbances inside the boundary layer.

Tadjfar, M.↗

NASA NTRS1993

Surface roughness due to residual ice in the use of low power deicing systems

Shin, Jaiwon↗

NASA NTRS1993

Multigrid methods for flow transition in three-dimensional boundary layers with surface roughness

The efficient multilevel adaptive method has been successfully applied to perform direct numerical simulations (DNS) of flow transition in 3-D channels and 3-D boundary layers with 2-D and 3-D isolated and distributed roughness in a curvilinear coordinate system. A fourth-order finite difference technique on stretched and staggered grids, a fully-implicit time marching scheme, a semi-coarsening multigrid method associated with line distributive relaxation scheme, and an improved outflow boundary-condition treatment, which needs only a very short buffer domain to damp all order-one wave reflections, are developed. These approaches make the multigrid DNS code very accurate and efficient. This allows us not only to be able to do spatial DNS for the 3-D channel and flat plate at low computational costs, but also to do spatial DNS for transition in the 3-D boundary layer with 3-D single and multiple roughness elements, which would have extremely high computational costs with conventional methods. Numerical results show good agreement with the linear stability theory, the secondary instability theory, and a number of laboratory experiments. The contribution of isolated and distributed roughness to transition is analyzed.

Liu, Chaoqun↗

NASA NTRS1993

A general rough-surface inversion algorithm: Theory and application to SAR data

Rough-surface inversion has significant applications in interpretation of SAR data obtained over bare soil surfaces and agricultural lands. Due to the sparsity of data and the large pixel size in SAR applications, it is not feasible to carry out inversions based on numerical scattering models. The alternative is to use parameter estimation techniques based on approximate analytical or empirical models. Hence, there are two issues to be addressed, namely, what model to choose and what estimation algorithm to apply. Here, a small perturbation model (SPM) is used to express the backscattering coefficients of the rough surface in terms of three surface parameters. The algorithm used to estimate these parameters is based on a nonlinear least-squares criterion. The least-squares optimization methods are widely used in estimation theory, but the distinguishing factor for SAR applications is incorporating the stochastic nature of both the unknown parameters and the data into formulation, which will be discussed in detail. The algorithm is tested with synthetic data, and several Newton-type least-squares minimization methods are discussed to compare their convergence characteristics. Finally, the algorithm is applied to multifrequency polarimetric SAR data obtained over some bare soil and agricultural fields. Results will be shown and compared to ground-truth measurements obtained from these areas. The strength of this general approach to inversion of SAR data is that it can be easily modified for use with any scattering model without changing any of the inversion steps. Note also that, for the same reason it is not limited to inversion of rough surfaces, and can be applied to any parameterized scattering process.

Moghaddam, M.↗